Preheat your oven to 275°F. Flip the ribs bone-side up and peel off the thin membrane. Use a butter knife to loosen a corner, then grip with a paper towel and pull off in one sheet. Mix all dry rub ingredients in a small bowl. Pat the ribs dry, coat all over with the rub, and press it in firmly on both sides. Place racks meat-side up on a large foil-lined rimmed baking sheet.
Cover the pan tightly with a second sheet of foil, crimping the edges to seal. Bake for 2 hours 45 minutes to 3 hours 15 minutes, until a knife slides easily between the bones. To check, lift a rack with tongs. It should bend and nearly crack in the middle. If it still feels stiff, reseal and bake another 15 minutes.
While the ribs bake, combine all BBQ sauce ingredients in a small saucepan. Bring to a simmer over medium heat, then reduce to medium-low. Cook 8 to 10 minutes, stirring frequently, until glossy and coating the back of a spoon. Remove from heat.
Remove the foil cover and switch the oven to broil on high. Brush the ribs generously with BBQ sauce. Broil 2 to 4 minutes until bubbling and caramelized. Brush with another layer of sauce and broil 1 to 2 minutes more. Watch closely, as the sugar can burn quickly.
Rest the ribs uncovered for 10 minutes, then slice between the bones. Serve with remaining BBQ sauce on the side.
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Notes
If the rack feels stiff when lifted with tongs, reseal the foil and bake another 15 minutes before checking again.Ribs can be rubbed and refrigerated uncovered overnight for deeper flavor.Store leftovers in an airtight container for up to 4 days.Reheat wrapped in foil at 300°F for 20 minutes, then brush with sauce and broil briefly to re-caramelize.